This invention relates to a process for preparing 4,6-diamino-1,3-benzenediols (4,6-diaminoresorcinols). More specifically, it relates to the preparation of 4,6-diaminoresorcinols starting with 1,3-benzenediols (resorcinols).
Diaminoresorcinols are used to prepare polybenzoxazoles (PBOs), the utility of which is discussed by, for example, Wolfe in Mark et al., The Encylopedia of Polymer Science and Engineering, Volume 11, pp. 601-635, Wiley-InterScience Publication, New York, 1988. One of the more efficient and economical methods of preparing 4,6-diaminoresorcinol is described by Lysenko (U.S. Pat. No. 4,766,244, herein incorporated by reference), which reports the synthesis of the desired product in high purities and yields in three steps from 1,2,3-trichlorobenzene.
Halogenated aromatic compounds have become the subject of close environmental scrutiny in recent years. Thus, the once inexpensive and plentiful 1,2,3-trichlorobenzene is becoming expensive and hard to obtain. The questionable long-term availability of 1,2,3-trichlorobenzene requires that an inexpensive commodity starting material be found to prepare 4,6-diaminoresorcinol.
Lysenko et al. addresses this problem somewhat in U.S. Pat. No. 4,982,001, herein incorporated by reference, by preparing 4,6-diaminoresorcinol from the inexpensive and readily available resorcinol through a 1,3-bis(methylcarbonato)benzene intermediate. The steric hindrance of this intermediate causes nitration to take place mostly at the 4- and 6-positions, so that the desired 4,6-diaminoresorcinol can be prepared upon hydrolysis and hydrogenation. Unfortunately, a significant degree of undesirable nitration occurs at the 2-position of the intermediate, and there is the danger of forming the potentially adversely reactive styphnic acid.
Zollinger et al. (Helvetica Chimica Acta, Volume XLI, pp. 1816-1823 (1958), herein incorporated by reference) reports that 4,6-bisphenylazoresorcinol can be prepared from resorcinol and benzenediazonium chloride under basic conditions, using 0.1 weight percent resorcinol based on the weight of a buffered aqueous solution.
This 4,6-bisphenylazoresorcinol can be reduced to the corresponding 4,6-diaminoresorcinol, as shown over 100 years ago by Liebermann and Kostanecki (Berichte der Deutschen Chemischen Gesellschaft, Volume XX, pp. 3114-3119 (1888), herein incorporated by reference).
Because the reaction between resorcinol and phenyl diazonium salts is so rapid, it is difficult to avoid making the undesirable side-product, 2,4,6-trisphenylazoresorcinol. Zollinger avoids this problem somewhat, though not entirely, by keeping the initial concentration of resorcinol at about 0.1 weight percent based on the weight of solvent. Though this approach may be practical for preparing 4,6-bisphenylazoresorcinols on a laboratory scale, process scale preparations require higher reagent concentrations than Zollinger contemplates. The challenge, then, becomes maintaining or improving upon, at concentrations above about 0.5 weight percent loading of resorcinol, the yield and selectivity of 4,6-bisphenylazoresorcinol achieved at lower concentrations.